An artificial hip joint has two main components; a prosthesis stem (also referred to as a femoral stem) and a cup. One end of the femoral stem is provided either with a spherical ball head or a prosthesis neck on which can be placed a ball head, where the ball head is designed for a close, sliding fit in a spherical recess of the cup. Together, the femoral stem with the ball head and the cup will act as a ball joint to replace the natural ball joint of the hip. The other end of the femoral stem comprises an elongated part designed to be attached to the hollow femoral canal in the patient's femur.
The cup is designed to be attached to a cavity in the patient's pelvis. A hemispherical shaped, recess in the cup is linked with an exterior surface designed to be attached to the pelvis via a side face. The exterior surface may have various shapes, all according to the method of attachment to the pelvis and other choices made by the supplier. Several of the cups that are in use are shaped as an approximate hemisphere, where the outer hemispherical surface is designed to be cemented to the pelvis. The side face that connects the recess and the exterior surface may be flat or possibly inwardly sloping towards the recess, which is preferably approximately centered in the side face. While the femoral stem and the cup may be fixed to the femur and the pelvis respectively using cement, a cement-free force fit also is possible.
When replacing a worn out hip with a prosthesis, the head of the femur is replaced. This is done by cutting the neck of the femur and hollowing out the top of the femoral canal in order to make room for the elongated femoral stem that is either cemented into the hole or force fitted. Further, the cavity on the pelvis is milled out to receive the cup, which is then fixed either by means of cement or a force fit. If the ball head is detachable, it is placed on the femoral stem before the ball head is placed in the cup, the joint is assembled by lifting the patient's leg up to a natural position and inserting the ball head in the recess in the cup, whereupon the incision is closed.
Conventional methods of positioning the implant (i.e., the cup and the femoral stem) focus on the range of motion of the artificial hip joint to find the optimal cup and stem position. More specifically, the surgeon, without navigation, will place the stem and cup and then perform a trial. At some point before the closure, the surgeon, to assess stability, will do impingement testing, shuck testing and will hold the hip in neutral flexion extension and ab/adduction, while internally rotating the hip a subjective 30-40 degrees. With the leg internally rotated, a well positioned implant should have the calcar of the trial implant entering the cup central to the apex of the cup. If the cup and stem combination is well placed in relation to the anteversion for both components, then the implant has the greatest chance of eliminating dislocation, and with eventual greater range of motion.
Modern navigation techniques allow precise positioning of the cup relative to the anterior pelvic plane (APP). The APP describes a safe zone for the relationship between the cup and the prosthesis stem with respect to the patient's hip geometry. However, such methods do not take into account the personal differences in hip mechanics and stability. Further, variations in pelvic tilt, for example, can affect the resulting spatial orientation of the cup.
For example, studies of pelvic tilt have shown that the average pelvic tilt at rest is about −4 degrees in the lying position and about −8 degrees in the standing position. Further, the pelvic tilt range between individuals, for example, may vary between −27 degrees and +3 degrees.
Pelvic reclination of 1 degree will lead to functional anteversion of the cup by approximately 0.7 degrees. Thus, navigation systems that reference the APP can be inaccurate due to pelvic tilt.